The present invention relates to a rod lens array in which a number of rod-shaped lens elements are arrayed in at least one row between two side plates, and clearances between the side plates and the rod-shaped lens elements are filled with resin to form a single integral unit. More specifically, the present invention relates to a rod lens array in which a glass plate is used as a side plate to improve the array accuracy (the alignment) of the lens elements using the flatness and smoothness of the glass side plate to thereby enhance resolution.
The rod lens array is a beam converging lens component of rectangular shape in which a number of refractive index rod-shaped lens elements each having a refractive-index distribution in the radial direction are arrayed in parallel in at least one row between two side plates disposed in parallel at a distance, and clearances between the side plates and the lens elements are impregnatingly filled with black silicone resin to form one continuous erected image of the same magnification as overlapping images respectively formed by the adjacent lens elements. Since the optical path is short, and the reversing mirror is not necessary, the use of a rod lens array can make apparatus small in size. For this reason, the rod lens array is widely used in a scanning optical system of a facsimile machine, a printer, etc.
The side plate is made of a black material that does not allow the beam to pass therethrough, because of the following reasons:
(1) it is necessary to render the coefficient of thermal expansion to be close to that of the lens material in order to prevent disorder of the array of the lens elements during heat process after the impregnation with resin;
(2) it is necessary to render the grindability to be close to that of lens material since the side plate and the lens elements are simultaneously ground;
(3) it is required to eliminate beam that pass through the portion other than the lens in order to maintain the resolution.
In general, fiber reinforced plastic (hereinafter referred to as FRD) is used, and specifically, a glass-cloth based epoxy resin laminated plate is used.
With ever-increasing miniaturization, sophistication, improvement in functionality, and drop in price of electronic equipment, the rod lens array used for an optical scanning system is also required to be miniaturized and thus the lens element is decreasing in diameter, and the requirement for optical performance of the rod lens array is also increasing.
In such increasing miniaturization, sophistication, and improvement in functionality, recently, occurrence of slight irregularities in alignment of the lens elements has been taken as a problem. Such slight irregularities in alignment of the lens elements cause periodic variations in resolution along the length of the rod lens array, and accordingly, a phenomenon of periodic variations in density occurs when it is used in a reading system/writing system for half tone processing.
From a study of the cause of such slight irregularities in alignment of the lens elements in the rod lens array of the related art, it was found to be periodical corrugations existing on the surface of the FRP corresponding to the arrangement of fiber bundles. Under the circumstances, when glass plates having preferable smoothness are used for side plates as a trial, it was certainly recognized that the irregularities in alignment of the lens element could be prevented. For example, when the positions of the lens elements were measured at the position 4 mm axially away therefrom, and the amounts of displacement were obtained for a hundred and forty lens elements, the standard deviation of displacement in the direction of alignment of the lens elements were 0.67 xcexcm when the FRP plates were used as the side plates, and 0.31 xcexcm when the glass plates were used.
However, there was still a problem remained in that it cannot put into actual use because resolution is lowered depending on beam transmitted through the glass plates. The MTF (Modulation Transfer Function) that is an indicator of resolution of the lens array can be expressed by the equation:
MTF=(Imaxxe2x88x92Imin)/(Imax+Imin)xc3x97100
where I max is a maximum value and I min is a minimum value of the intensity I obtained when reading a monochrome pattern. Therefore, since beam transmitted through the glass increases the value of I min, resolution is accordingly lowered.
Accordingly, an object of the present invention is to provide a rod lens array adapted to increase the accuracy of alignment of the lens elements at low cost and to prevent lowering of resolution. Another object of the present invention is to provide a rod lens array mounting body in which the accuracy of alignment of the lens elements can be increased and lowering of resolution can be prevented.
The present invention is a rod lens array in which a number of rod-shaped lens elements are arrayed in at least one row between two side plates, and the clearances are filled with resin to form a single integral unit. In the present invention, two side plates are formed of glass plates, the inner surfaces of these two glass plates facing the lens elements are flat and smooth, the outer surfaces on the opposite sides are formed with reflection-preventing portions, the side surfaces of both of the glass plates on the beam-exit side are formed with beam-shielding zones from the outer edges inwardly along almost the whole length, and the width of the beam-shielding zone Ts satisfies the following relation:
Tg greater than Ts greater than Tgxe2x88x92Zxc3x97Dxc3x97{2+{square root over (3)}xc3x97(nxe2x88x921)}/{2xc3x97(TCxe2x88x92Z)},
where:
Tg: thickness of the glass plate
D: diameter of lens element
Z: length of lens element
TC: conjugate length (face-to-face dimension of the object image)
n: number of columns of lens element.
Instead of the construction in which the beam-shielding zone is provided, the thickness of both of the glass plates Tg may be decreased as:
Tg greater than Zxc3x97Dxc3x97{2+{square root over (3)}xc3x97(nxe2x88x921)}/{2xc3x97(TCxe2x88x92Z)}.
In the present invention, xe2x80x9creflection-preventing portionxe2x80x9d means a processed layer for preventing reflection from occurring to the extent that little reflection is assumed to occur on the outer surface of the glass plate, or a processed layer that causes a scattering of light. In the construction described above the reflection-preventing portion includes an anti-reflection coating layer formed on the outer surface of the glass plate, a rough surface formed with fine pits and projections, or a refractive index matching portion.
As described thus far, it is preferable to subject the glass plate to be used as a side plate to anti-reflection processing in itself. However, anti-reflection processing may be applied to a holder member that is to be brought into contact with the glass plate, or when joining them together. Alternatively, a beam-shielding zone may be formed by the use of the holder member that covers the glass plate instead of forming a beam-shielding zone on the glass plate in itself.
The present disclosure relates to the subject matter contained in Japanese patent application No. 2000-111257 (filed on Apr. 12, 2000), which is expressly incorporated herein by reference in its entirety.